The present invention relates to a method for producing a polymer.
The classical parameters which are relevant when setting up a polymerization process are the composition of the polymer, the reaction temperature, the viscosity, the reaction pressure, concentration ratios of the reactants, the pH in the case of aqueous systems, the molecular weight distribution and the particle size of a heterophase polymer. Other process-relevant quantities derivable from the process are the heat transfer coefficient, the mixing time, the degree of dispersing, shear load and maximum shear stress and the power input. It should be understood here that the first-mentioned parameters rather characterize the polymeric product while the process-relevant quantities on the other hand describe and characterize the process and the process control. Reliable process control, in particular in relatively large reaction containers, ensures that the reaction product is obtained exactly with said and required product properties.
EP 1 163 504 B1 discloses a method for producing latex by emulsion polymerization, the online monitoring/control being effected by means of Raman spectroscopy. The data obtained on the basis of the Raman spectra are compared with specific reference data and, starting from this comparison, the reaction parameters are controlled so that a deviation of the measured data from the reference data is minimized. In particular, temperature, pressure, movement of the medium and metering of the monomer are mentioned as reaction parameters.
U.S. Pat. No. 6,657,019 B2 discloses a method for predicting polymer latex properties in a polymerization method, in which a set of parameters of the method is measured and is evaluated on the basis of heat and mass balance and the data obtained are compared with a set of predetermined data and statistical relationships between the parameters of the method in order thus to predict the properties of the polymer.
U.S. Pat. No. 6,991,763 B2 discloses a method for controlling the monomer level in a polymerization reaction. This method is based on a calorimetric measurement in which the cooling medium temperature at the entrance of the cooling jacket and at the exit of the cooling jacket and the flow rate of the cooling medium are measured. The heat transfer is determined from these values and is compared with a target value for the heat output. Starting from this, the monomer feed is regulated.
Waβmer et al., in “A Unified Model for the Mixing of Non-Newtonian Fluids in the Laminar, Transition and Turbulent Region”, Macromol. Mater. Eng. 2005, 290, 294-301, are concerned with the power input calculation in the case of structurally viscous systems comprising fluids for which a Reynolds number cannot be calculated directly. It is found that the fluid behavior of non-Newtonian fluids has a considerable influence on the calculation of process-relevant parameters, in particular power input, mixing time and heat transfer. A relationship between shear stress and specific power input is described. Furthermore, it is stated that the effective shear rate in the transition region between laminar and turbulent in a vessel and the stirrer speed are not linearly dependent.
U.S. Pat. No. 4,833,180 discloses a method for producing polyvinyl chloride, in which the polymerization process is adjusted so that a certain shear rate (between a paddle stirrer and a deflector) is achieved. A relationship between shear rates and coagulum formation or variation of the shear rates for avoiding coagulum formation is not discussed.
K. Takahashi et al., in “Mixing performance experiments in impeller stirred tanks subjected to unsteady rotational speeds”, Chem. Engineering Science, Vol. 53, No. 17, p. 3031-3040 (1998), show the influence of different stirring profiles on the speed of a decolorization reaction in a highly viscous, homogeneous polymer solution. From the results, it is concluded that the increase in the turbulence of the liquid phase due to the non-stationary stirring conditions leads to an improvement in the mixing effect. The method presented comprises a decolorization reaction in a transparent solution which permits the required changes in the stirring conditions without problems. However, the discoveries cannot be applied to disperse systems which are not transparent, frequently have shear-dependent viscosity values and show shear-dependent coagulation.
It is an object of the invention to provide a method for producing polymers, in which in particular the heat removal is optimized and in which no coagulation of the polymer particles produced occurs. A deterioration in the product properties is to be avoided.